The present invention relates to a field ionizer device and more specifically to a carbon nanotube field ionizer device.
Atoms or molecules can become field ionized in the presence of strong electric field in the proximity of a sharp tip biased positive relative to a cathode, yielding positive ions. [R. Gomer, Field emission and field ionization, Harvard Univ. press, 1961, Cambridge, Mass.]. Electrons tunnel from the atom or molecule into the tip, followed by the ejection of the resulting positive ion from the tip region.
The first work on field ionization was performed by Muller in 1953 [E. W. Muller, Ergeb. Exakt. Naturwiss. 27, 290 (1953)] who used a sharp single crystal tungsten needle biased positively to thousands of volts. It was observed that closer the molecule is to the tip surface (such as in the adsorbed state), the lower the field required for field ionization and the narrower the ion energy distribution becomes [Gomer]. In addition, the more polarizable the molecule, the higher is the probability of field ionization, due to the longer time the molecule spends in the ionization zone as a result of the attraction. This also applies to molecules in a liquid film state on the tip surface as they spend longer time than a gas in the ionization zone.
A natural application of field ionization is the ion source for mass spectrometry. The unique advantage here is that field ionization produces no internal excitation (vibrational, electronic) which leads to very little breakup of the molecule, whereas the more conventional electron impact ionization causes fragmentation of the parent molecule.
For example, field ionized acetone has just one major mass peak, whereas electron impact ionization produces 18 mass fragment peaks [Gomer]. The “soft” field ionization process vastly simplifies mass spectroscopic analysis, and would be of great value for the detection of large molecules such as proteins and other biological molecules.
In order to increase the ion currents, many field ionizer arrangements commonly using large and dense arrays of sharp needles, whiskers, and even nanorods have been used. These all do not have microfabricated gates and usually require high voltages. High voltages result in high molecular ion energies which is undesirable due to their fragmenting upon impact with the ion collector.
Microfabricated field ionizers with microns-size diameter integrated gates have been previously fabricated to greatly reduce the voltage for field ionization [C. A. Spindt, Surface Science 266, 145 (1992); B. Ghodsian, et al, IEEE Electron Device Letters 19, 241 (1998)]. Spindt's “microvolcano” field ionizer array used a microfabricated hollow, volcano-shaped metal cone through which the gas was injected from the backside. Some aspects of gas injection from the backside and forcing all the gas to flow through the ionizer tip can be found in the prior art. However, the present invention differs dramatically and offers advantages over Spindt's microvolcano ionizer as the carbon nanotubes can be on the sidewalls of the field ionizer apertures, which, due to their extreme sharpness and high aspect ratio, allows for lower voltages. Another aspect of the present invention is that the carbon nanotubes have a larger capacity and higher surface-to-volume ratio for adsorbing gases than the metal film in Spindt's microvolcanos. Field ionization efficiency is greatly enhanced by the higher surface concentration of gases (due to higher adsorption) in the ionization zone. The large surface-to-volume ratio of the nanotubes also facilitates allows liquid analytes to wick up the nanotube into the ionization zone in high molecular concentration of the liquid form.